docs: def set_dtreat_indlamb(self, indlamb=None):
          """ Store the desired index array for the wavelength
  
          If None => all wavelengths
          Must be a 1d array
  
          """
          if not self._isSpectral():
              msg = "The wavelength can only be set with DataSpectral object !"
              raise Exception(msg)
          if indlamb is not None:
              indlamb = np.asarray(indlamb)
              assert indlamb.ndim==1
              indlamb = _format_ind(indlamb, n=self._ddataRef['nlamb'])
          self._dtreat['indlamb'] = indlamb
          self._ddata['uptodate'] = False